Spatial localization of saccade targets. II. Activity of superior colliculus neurons preceding compensatory saccades.

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Abstract
Rhesus monkeys were trained to look to brief visual targets presented in an otherwise darkened room. Single-unit activity was recorded from the superior colliculus (SC) during the performance of 2 tasks for which successful completion was contingent on the combination of retinal-error and eye-position signals. The 1st task required successive eye movements to serially flashed targets, each of which was extinguished prior to the initial eye movement. In the 2nd task, electrical stimulation of the SC, delivered during the interval between target offset and saccade onset, drove the eyes to another position in the orbit, thereby requiring a compensatory saccade. The major purpose of this experiment was to determine whether or not neurons in the SC discharging before visually triggered saccades also discharge before saccades compensating for stimulation-induced perturbations in eye position. Tungsten microelectrodes were introduced into the SC bilaterally. Extracellular unit activity was monitored until a cell with saccade-related activity was isolated in one colliculus and stimulation of the opposite colliculus elicited an eye movement. After a movement field was plotted for the unit, its behavior during the double-saccade and compensatory saccade tasks was evaluated. Target coordinates were arranged such that the vector of the second or compensatory movement was in the movement field of the particular unit under investigation. Of the 50 cells studied 49 discharged before all saccades in their movement field, regardless of the type of trial that produced the saccade. The 1 neuron that did not fire before saccades compensating for the stimulation-induced movement required prior activation by a visual stimulus within its movement field. Burst of activity occurring before compensatory saccades on stimulation trials was usually less vigorous than the burst preceding control saccades to a direct visual target. Results are interpreted as supporting the hypothesis that the intermediate layers of the SC contain a spatial map of motor error, the vector of the saccade required to move the eyes to a desired orbital position. This implies that input signals reaching the intermediate layers of the SC may also be encoded in motor coordinates and that the SC may represent a site where signals from different sensory modalities were translated into a common motor frame of reference.

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